A persistent problem common to virtually all petroleum products and petroleum-related products is their tendency to undergo oxidative degradation. Oxidation may occur even under the relatively mild conditions attending storage and transport, and is appreciably accelerated when operating conditions are conducive to oxidation processes, for example, the elevated temperatures experienced by lubricating oil. Such oxidative processes not only cause chemical degradation of the petroleum or petroleum-related product, but may also cause appreciable changes in desirable physical properties, such as viscosity, which lead to a deterioration of product performance characteristics. Additionally, the oxidative products themselves may attack materials in contact with the petroleum and petroleum-related products, such as metals in contact with transmission or lubricating oils, thereby causing inefficient performance and, in extreme cases, even mechanical failure.
The class of N,N'-dialkyl-4,4'-diaminodiphenyl ethers is known to have substantial antioxidant properties, and has found utility as an additive protecting petroleum and petroleum-related products against oxidation in their working environment as shown in U.S. Pat. No. 2,982,729. Unsubstituted 2,4'-diaminodiphenyl ether acts as an effective stabilizer against oxidative deterioration, U.S. Pat. No. 2,910,437, and mixtures of alkylated 4,4'- and 2,4'-diaminodiphenyl ethers act synergistically as an inhibitor according to U.S. Pat. No. 2,964,479. It now has been found that certain diimines of 2,4'-diaminodiphenyl ethers are effective oxidants in the aforementioned products. In some cases these ethers display antioxidant properties exceeding those of the symmetrical diaminodiphenyl ethers, thereby permitting their effective use at relatively lower levels. The antioxidants described herein possess the further advantage that structural changes within broad, but nonetheless well-defined, limits are possible, thereby permitting optimization of the antioxidant for a particular product in a specified use.